Emphasize "should" a la rfc2119

src/main/java/org/apache/commons/collections4/Transformer.java

@@ -23,7 +23,7 @@
  * object into another.
  * <p>
  * A {@code Transformer} converts the input object to the output object.
- * The input object should be left unchanged.
+ * The input object SHOULD be left unchanged.
  * Transformers are typically used for type conversions, or extracting data
  * from an object.
  * </p>
@@ -49,7 +49,10 @@ default R apply(final T t) {
     }
 
     /**
-     * Transforms the input object (leaving it unchanged) into some output object.
+     * Transforms the input object into some output object.
+     * <p>
+     * The input object SHOULD be left unchanged.
+     * </p>
      *
      * @param input  the object to be transformed, should be left unchanged
      * @return a transformed object